1. Field of the Invention
This invention relates to the measurement of noise in field effect transistors (FETs), and more particularly to the measurement of noise for FETs operated toward the upper end of the microwave frequency range.
2. Description of the Related Art
With the exception of masers, the FET family of transistors are the lowest noise active devices presently known. Masers are expensive, cumbersome, narrow band, bulky because of the necessary cryogenic equipment, and in general so difficult to work with that they are not presently sold commercially. FETs on the other hand have the advantages of wide bandwidth, stable operation, ease of monolithic integration in integrated circuits, high efficiency and versatility of applications. Their low noise property is particularly important in satellite communication applications, and can also be useful in other applications such as radar, microwave links, and optical or infrared receivers. In each of these applications the ability to measure the noise parameters of the FET in its circuit can be used for device screening and selection of circuits that meet the particular specifications of the device, or for diagnostic purposes.
The generally known prior methods of making noise measurements in FETs are reviewed in National Bureau of Standards Monograph No. 142, U.S. Department of Commerce, 1974. A volume on the state-of-the-art for determining noise in FETs which includes reprints of significant publications on the subject is H. Fukui, "Low-Noise Microwave Transistors and Amplifiers", IEEE Press, New York 1981.
Integrated circuit FETs employed in amplifier circuits and the like are formed on thin semiconductor wafers, with the FET circuit replicated on the wafer thousands of times. With the conventional technique for measuring FET noise, the wafer is first diced into small separate chips of one FET each, and each individual FET is then mounted and wire bond connected in a circuit. Unlike testing at the wafer stage prior to dicing when all devices on the wafer can be tested with only one wafer handling operation, the separate devices must be individually handled in the noise testing procedure because they have already been mounted and bonded. Since the yield of high-performance devices is low (particularly for mm wave devices, which have such small dimensions that their characteristics change significantly with the etching of even a couple of atomic layers during fabrication), the individual mounting, bonding and testing steps carried out on rejected devices are wasted.
The conventional test procedure itself is a very lengthy and time-consuming process. For example, one important noise parameter is F.sub.min, which represents the minimum noise figure for the FET (under the circumstances when it is connected in an optimum circuit). In one testing procedure the device is mounted in a microwave circuit which is operated at the operating frequency of interest, and F.sub.min is measured either by making four or more measurements of the noise figure, and the corresponding source impedances at the FET input, or by a trial and error approach which involved manually tuning the circuit while observing a noise meter, and taking the minimum noise reading as F.sub.min. A complicating problem is that there are many different ways to tune the circuit, all of which must be done slowly and carefully. The process is so laborious and time-consuming that it is suitable only for laboratory applications. Other attempts to avoid these shortcomings have involved even more elaborate measurement setups. See K. Froelich, "Measurement of GaAs FET Noise Parameters", Watkins Johnson Co. Tech. Notes, Vol. 13, No. 6, pages 2-11, Nov./Dec. 1986; R. Q. Lane, "Derive Noise and Gain Parameters in 10 Seconds", Microwaves, Vol. 17, No. 8, pages 53-57, August 1978.
In the other method of measurement, F.sub.min is derived from a measured value of the noise figure F and the admittance Y.sub.g of the generator or signal source presented to the FET during measurement at the operating frequency. The minimum noise figure F.sub.min can be deduced from the data only if four or more values of F are measured for four well-separated values of Y.sub.g. In addition to the difficulty of tuning Y.sub.g in a probe station set-up, a measurement of Y.sub.g must also be carried out at the low measurement frequency.
An ability to test for FET noise while still at the wafer stage would be very desirable, and could result in substantial savings. However, there are a number of obstacles which have not been overcome. With current testing equipment capable of handling entire wafers, the probes used for accessing individual devices contribute parasitic impedances to the measurement. These can be made tolerable at UHF and low microwave frequencies (the microwave range is generally taken to be about 1-100 GHz). However, at the upper end of the microwave frequency range (above about 20 GHz), the probes have a significant or even dominant effect upon the value of the measured noise figure (F).
Another problem with any attempt to test at wafer stage with present techniques is that, if the noise figure F is measured at a sufficiently low frequency at which the probe station parasitic effects are small, after F.sub.min and the other noise parameters have been deduced at the low frequency a method must be devised for predicting them from the F.sub.min of the device at them much higher operating frequency. Furthermore, experimental measurements of the noise figure of mm wave MESFET devices as a function of frequency have shown that at low frequency (typically below a few GHz for 0.25 micron gate-length devices), the measured noise figure is strongly influenced by the circuit losses. As a result, the measured F.sub.min at the low frequency is not a direct measure of the device capability at the higher operating frequency, and cannot be used to predict the high frequency device performance. Attempts to reduce circuit losses at the low frequency are again stymied by the wafer handling probe station.